The present invention relates to a typing mechanism for a serial printer suitable for use in the display of desk-top calculators, measuring instruments and so forth.
Many desk-top calculators, measuring instruments and so forth proposed heretofore employ displays using liquid crystals or the like for producing a digital display. Recently, however, there have been proposed desk-top calculators and measuring instruments having a small serial printer for printing and recording on recording paper the result of calculations or measurments. The serial printers of the type described preferably have as small a size as possible because of limited space. It is often quite difficult to realize the small size desired because the serial printer has various complicated mechanisms, such as the mechanism for rotatably driving the type wheel, the mechanism for shifting the carriage, and the mechanism for driving the typing hammer, paper feed mechanism and so on.
Current desk-top calculators have various operation functions, and it is required that the calculators be able, in addition to printing the numerals 0 to 9, to print various data on the operation function, and the number of these operation functions may well correspond to that of the numeral data. If the numerals and function symbols are carried by a single type wheel, the size of the type wheel must be increased to an impractical size so that the size of the printer would be increased inconveniently to make the printer unsuitable for use in portable type desk-top calculators or the like.
In order to obviate this problem, a new serial printer has been proposed in which the type wheel for the function symbol and the type wheel for numerals are constructed separately from each other and only the type wheel having the numerals is shifted to provide the different columns of characters. As can be seen from Japanese Patent Laid-open Publication No. 46930/1977, this type of serial printer has two hammers which are associated with two type wheels and operate independently of each other. This arrangement requires a complicated mechanism for selectively actuating the two hammers by a single actuating mechanism.
Also a serial printer has been proposed in which the mechanism for selectively actuating the two hammers is eliminated and two type wheels are associated with a single common hammer so as to be selectively operated by the latter. Such a serial printer is shown in Japanese Patent Laid-open Publication No. 56514/1978.
The serial printers shown in Japanese Patent Laid-open Nos. 46930/1977 and 56514/1978 are suitable for use in desk-top calculators or the like, because they have sufficiently reduced size and weight. However, the cost of parts of the driving power source and the space occupied by the latter tend to become large, because a pulse motor is used for rotatively driving the type wheels, a solenoid plunger of a comparatively large size is used for column shift of the type wheel and driving of the hammer, and another solenoid plunger of a comparatively large size is used for feeding the paper and releasing the carriage.
On the other hand, Japanese Patent Laid-open No. 68325/1979 proposes a serial printer in which a single motor performs various actions such as selection of type, printing, column shift, paper feed and so forth. In this printer, however, there is a practical limit in increasing the typing speed, because the motor has to be reversible. For the same reason, the motor is comparatively expensive and requires a complicated control. In addition, a complicated mechanism is required for selecting and retaining the two type wheels independently.